The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
An optical disk, such as a CD, DVD, HD-DVD, or Blu-Ray disk, may contain data recorded as marks and spaces on the surface of the optical disk. The marks and spaces may correspond to bits of value ‘0’ and ‘1’ which comprise the data. After being recorded, the data may be read by detecting laser light reflected off the optical disk. The reflected light may be transformed by a photo-detector to an analog replay signal. When the data is written, each mark and space are intended to be the same nominal size on the optical disk. However, different types of optical disks and variations in the power and focus of the recording laser may cause asymmetry, where the sizes of the marks and spaces differ from their intended nominal sizes. For example, if the writing power is too small, then the mark size can be shorter than the nominal size, and the space size can be longer than the nominal size. When reading the marks and spaces, asymmetry in the replay signal may result in amplitude and duration variations and an increase in the bit error rate.
Examples of normal and distorted signals are shown in FIG. 1A and FIG. 1B, respectively. As illustrated in FIG. 1A, an ideal signal may exhibit near-identical durations of ‘1’ and ‘0’ bits. An asymmetric signal as depicted in FIG. 1B, in contrast, would tend to exhibit compressed or rarified amplitude widths, corresponding to shorter durations of a bit relative to a ‘0’ bit, or vice-versa. The resulting asymmetric signal may have the same data content (same peaks and valleys as the ideal signal), but may instead contain amplitude variations that appear shifted downwards in the y-axis, since the data signal may be spending more time at the longer ‘0’ bit than at the shorter ‘1’ bit. A further complication may arise if there is a D.C. or other low frequency offset component biasing the data signal (signal “offset,” FIG. 1B). This could result in an undesired shift to the entire signal in addition to the signal asymmetry.